Spatiotemporal evolution of the heterogeneous oxide layer and ablation performance of C/C-HfC composites: A combined experimental and numerical study

The protective efficacy of hafnium carbide (HfC) coatings on Carbon/Carbon (C/C) composites is closely linked to the dynamic evolution of their oxide layer during ablation. While the dense hafnium oxycarbide (HfC_1-xO_x) acts as an effective barrier, its further oxidation into the loose HfO₂ leads to the degradation of the protective performance of the coatings. This study extends a composition-evolution ablation model by explicitly resolving surface topography to investigate the spatiotemporal evolution of the heterogeneous oxide layer and the protective performance during ablation. The proposed ablation model is validated against experimental measurements of elemental (C, O, and Hf) distributions after ablation. The results reveal a two-stage ablation mechanism: an initial protective stage maintained by the growth of dense HfC_1-xO_x, followed by an accelerated degradation stage triggered by the accumulation of porous HfO₂. This study also demonstrates that the geometric effect leads to a concentrated oxygen flux in the valley regions, resulting in a more rapid consumption of HfC compared to the peak regions. Furthermore, an elevated oxygen mole fraction (from 20% to 60%) accelerates the degradation of the oxygen barrier properties, causing an approximately 11% reduction in its optimal performance. These insights provide a basis for the reliability analysis and lifetime prediction of HfC-based ablation-resistant coatings.